The present invention relates to a machining tool for an aircraft fuselage frame.
As shown in FIG. 1, a fuselage frame 10 comprises a profile having a Z-shaped cross-section whose central portion, referred to as a core 12, forms a complete or partial ring. The profile comprises a first wing 14, referred to as an inner wing, arranged in the area of the inner edge of the core 12 and perpendicular to the latter, and a second wing 16 referred to as an outer wing, arranged in the area of the outer edge of the core 12, also perpendicular to the latter.
This Z-shaped profile follows, in a plane containing the core 12, a radius of curvature. Thus, the two wings 14 and 16 correspond approximately to a portion of a cylinder.
As shown in FIG. 2, as a function of its position in the structure of the fuselage, a frame can be more or less long and the radius of curvature of the wings can be more or less pronounced. Thus, in FIG. 2, an example of a frame is shown in a front view in a continuous line, and other frames with different shapes are shown in dotted form, also in a front view.
According to an embodiment, an aircraft fuselage frame can be made from a composite material and comprise reinforcement fibers embedded in a resin matrix.
After polymerization, a fuselage frame must be subjected to machining operations consisting of routing and then beveling the inner wing 14, routing the outer wing 16, piercing the core 12, and finally cutting the ends of the frame.
For these machining operations to be carried out, the fuselage frame is affixed to a tool arranged in the machine tool unit.
For these machining operations to be carried out while respecting dimensional tolerances, the tool comprises at least one reference surface against which one of the surfaces of the core is flattened and at least one second reference (surface or abutment) against which the inner wing takes support.
According to a first alternative, the tool comprises a first reference surface against which the core takes support and which extends over the entire length of the core, and a second reference surface against which one of the surfaces of the inner wing takes support and which extends over the entire length of the inner wing.
The clamping of the frame on the tool is obtained by a “suction” type clamp. Thus, the reference surfaces comprise depression zones with joints to maintain the core and the inner wing flattened against the first reference surface and against the second reference surface respectively.
After the clamping of the frame to be machined, the different machining operations are carried out. At the end of the machining operation, the operator detaches the machined frame from the tool and cleans the machine tool unit.
To proceed with machining another frame, if the latter has the same geometry as previously described, the operator clamps it on the same tool and restarts a machining phase. If the frame has a different geometry, it is necessary to disassemble the tool in order to install a tool adapted to the frame to be made.
Therefore, according to this first alternative, as many tools as there are frames must be provided. Thus, to be able to machine the frames of different portions of an aircraft fuselage, and thus for the entire range of aircrafts, it is necessary to have a hundred or so different tools, which necessarily has an impact on the manufacturing and storage costs of the tools. In addition, the multitude of tools complicates the management thereof.
According to another drawback, the setting in place of the frame on the tool is a problem because it is difficult to flatten, simultaneously and over the entire length of the frame, the core and the inner wing against the first reference surface and against the second reference surface, respectively, the connection between the frame and the tool being redundant.
According to this alternative, each reference surface comprises zones isolated from one another in the area of which the depression phenomenon is activated, sequentially and gradually as the setting in place is taking place.
To ensure a correct set up over the entire length, the frame must be correctly positioned in the area of the first activated zone. If it is not correctly positioned at the beginning, the defect in the alignment becomes amplified over the length.
The positioning of the frame on the tool is all the more difficult as the joints which border the depression zones have a substantial friction coefficient with the frame and limit gliding.
According to a second alternative, the reference surface of the core is not made in one piece. Therefore, the tool comprises several blocks positioned on a horizontal machining table, each comprising a reference surface provided to be used to support the core.
To ensure the positioning of the frame along the plane of the core, the core is secured onto one of the blocks and the inner wing takes support locally against at least one abutment affixed to another block.
In order to position the frame correctly, it is necessary to use 3 to 7 blocks that can be reused by positioning them correctly for the different geometries of the frames.
According to this alternative, the blocks are set in plate on the table with the head of the machine tool unit and are affixed to the table by a “suction” type clamping system. One of the surfaces of the frame core is attached to the reference surfaces of the different blocks, then the frame is secured onto one of the blocks. Subsequently, one of the surfaces of the inner wing is placed in contact locally with at least one abutment affixed to at least one block. Finally, the frame is affixed to the blocks by a “suction” type clamping system.
After machining, the frame is disassembled.
To proceed with the machining of another frame, if the latter has the same geometry as the previous one, the operator fits it on the blocks and re-starts a machining phase. If the frame has a different geometry, the blocks must be re-positioned on the table.
Even if this alternative makes it possible to reduce the number of tools, it is not fully satisfactory for the following reasons:                Insofar as the blocks are set in place with the assistance of the head of the machine tool unit to position them correctly, their setting in place impacts the productivity of the machine tool unit. The latter is all the more impacted as, according to this alternative, the number of elements to be positioned is greater than for the first alternative.        The piercings to be made in the core must be made with reduced feed rates to limit the delamination risks because for the piercings located outside the blocks, there is no support surface on the periphery of the piercings to take over the piercing forces. Consequently, productivity is further impacted.        According to another drawback, using a horizontal machining table and arranging the plane of the core along a horizontal plane leads to the machining surfaces to be contaminated by the cutting fluid or residues from machining        As for the first alternative, the setting in place of the frame on the blocks is difficult.        In addition to the difficulty of the setting in place, if the operator forces to defect in the alignment to be corrected, the frame to be machined can be secured on the blocks with stress, particularly bending stress between the blocks. However, if the frame is machined while it is stressed, it can slightly deform to free the stress when it is unsecured. Pursuant to these deformations, certain dimensional tolerances may no longer be respected. Therefore, according to the second alternative, the beveling operation must be performed in another unit, which hinders productivity.        
Thus, the present invention aims at overcoming the drawbacks of the prior art while improving productivity.